Unity State Machine Patterns

Overview

State machines manage game flow, UI navigation, and entity behavior. This skill covers safe implementation patterns for async state transitions and proper cleanup.

When to Use

• Game state management (menu, loading, gameplay)
• UI navigation flows
• AI behavior states
• Animation state control
• Feature/activity controllers

Basic State Machine

State Interface

public interface IState
{
    UniTask Enter(IStateTransitionData data);
    UniTask Exit(bool applicationExiting);
    UniTask Suspend(IStateTransitionData nextStateData);
    UniTask Resume();
    IStateTransitionData GetTransitionData();
}

public interface IStateTransitionData
{
    Type Installer { get; }
}

Simple State Implementation

public abstract class BaseState : IState
{
    protected bool IsActive { get; private set; }
    protected bool IsSuspended { get; private set; }
    
    public virtual async UniTask Enter(IStateTransitionData data)
    {
        IsActive = true;
        IsSuspended = false;
        await OnEnter(data);
    }
    
    public virtual async UniTask Exit(bool applicationExiting)
    {
        if (!IsActive) return;
        
        IsActive = false;
        await OnExit(applicationExiting);
    }
    
    public virtual async UniTask Suspend(IStateTransitionData nextStateData)
    {
        IsSuspended = true;
        await OnSuspend();
    }
    
    public virtual async UniTask Resume()
    {
        IsSuspended = false;
        await OnResume();
    }
    
    protected abstract UniTask OnEnter(IStateTransitionData data);
    protected abstract UniTask OnExit(bool applicationExiting);
    protected virtual UniTask OnSuspend() => UniTask.CompletedTask;
    protected virtual UniTask OnResume() => UniTask.CompletedTask;
    
    public abstract IStateTransitionData GetTransitionData();
}

State Navigation Controller

Core Navigation Controller

public class StateNavigationController : IStateNavigator
{
    private readonly LifetimeScope _rootScope;
    private readonly List<LifetimeScope> _stateStack = new();
    private bool _navigationInProgress;
    
    public StateNavigationController(LifetimeScope rootScope)
    {
        _rootScope = rootScope;
    }
    
    public async UniTask NavigateTo(IStateTransitionData transitionData)
    {
        // Wait for any ongoing navigation
        if (_navigationInProgress)
        {
            var waited = await UniTask.WaitUntil(() => !_navigationInProgress)
                .TimeoutWithoutException(TimeSpan.FromSeconds(5));
            
            if (!waited)
            {
                Debug.LogError("Navigation timeout - forcing through");
                _navigationInProgress = false;
            }
        }
        
        _navigationInProgress = true;
        
        try
        {
            // Exit all current states
            await ExitAllStates();
            
            // Enter new state
            await EnterState(transitionData);
        }
        finally
        {
            _navigationInProgress = false;
        }
    }
    
    public async UniTask NavigateToAdditive(IStateTransitionData transitionData)
    {
        if (_navigationInProgress)
        {
            await UniTask.WaitUntil(() => !_navigationInProgress);
        }
        
        _navigationInProgress = true;
        
        try
        {
            // Suspend current state
            if (_stateStack.Count > 0)
            {
                var currentScope = _stateStack[^1];
                if (TryResolveState(currentScope, out var currentState))
                {
                    await currentState.Suspend(transitionData);
                }
            }
            
            // Enter new state additively
            await EnterState(transitionData);
        }
        finally
        {
            _navigationInProgress = false;
        }
    }
    
    public async UniTask ExitActiveState()
    {
        if (_navigationInProgress || _stateStack.Count == 0)
            return;
        
        _navigationInProgress = true;
        
        try
        {
            // Pop and exit top state
            var exitScope = PopState();
            
            if (TryResolveState(exitScope, out var exitState))
            {
                await exitState.Exit(false);
            }
            
            // CRITICAL: Wait before disposal
            await UniTask.Yield();
            
            exitScope.Dispose();
            
            // Resume underlying state
            if (_stateStack.Count > 0)
            {
                var revealedScope = _stateStack[^1];
                if (TryResolveState(revealedScope, out var revealedState))
                {
                    await revealedState.Resume();
                }
            }
        }
        finally
        {
            _navigationInProgress = false;
        }
    }
    
    private async UniTask EnterState(IStateTransitionData transitionData)
    {
        var installer = (IInstaller)Activator.CreateInstance(transitionData.Installer);
        var stateScope = _rootScope.CreateChild(installer);
        
        _stateStack.Add(stateScope);
        
        if (TryResolveState(stateScope, out var state))
        {
            await state.Enter(transitionData);
        }
    }
    
    private async UniTask ExitAllStates()
    {
        while (_stateStack.Count > 0)
        {
            var scope = PopState();
            
            if (TryResolveState(scope, out var state))
            {
                await state.Exit(false);
            }
            
            await UniTask.Yield();
            scope.Dispose();
        }
    }
    
    private LifetimeScope PopState()
    {
        var index = _stateStack.Count - 1;
        var scope = _stateStack[index];
        _stateStack.RemoveAt(index);
        return scope;
    }
    
    private bool TryResolveState(LifetimeScope scope, out IState state)
    {
        state = null;
        try
        {
            if (scope?.Container == null) return false;
            state = scope.Container.Resolve<IState>();
            return state != null;
        }
        catch
        {
            return false;
        }
    }
    
    public async UniTask Shutdown()
    {
        for (int i = _stateStack.Count - 1; i >= 0; i--)
        {
            var scope = _stateStack[i];
            
            if (TryResolveState(scope, out var state))
            {
                await state.Exit(true);
            }
            
            scope.Dispose();
        }
        
        _stateStack.Clear();
    }
}

Activity State Pattern

Activity State Base

public abstract class ActivityState<TController> : IState 
    where TController : IActivityController
{
    private readonly TController _controller;
    private bool _disposed;
    
    protected ActivityState(TController controller)
    {
        _controller = controller;
    }
    
    public async UniTask Enter(IStateTransitionData data)
    {
        if (_disposed) return;
        await _controller.Initialize(data);
    }
    
    public async UniTask Exit(bool applicationExiting)
    {
        if (_disposed) return;
        _disposed = true;
        
        await _controller.Shutdown(applicationExiting);
    }
    
    public async UniTask Suspend(IStateTransitionData nextStateData)
    {
        if (_disposed) return;
        await _controller.Suspend(nextStateData);
    }
    
    public async UniTask Resume()
    {
        if (_disposed) return;
        await _controller.Resume();
    }
    
    public IStateTransitionData GetTransitionData()
    {
        return _controller.GetTransitionData();
    }
}

Activity Controller Base

public abstract class ActivityController<TData, TView> : IActivityController
    where TData : IStateTransitionData
    where TView : ActivityView
{
    protected TData TransitionData { get; private set; }
    protected TView View { get; private set; }
    protected CancellationTokenSource LifecycleCts { get; private set; }
    
    private bool _isShuttingDown;
    
    public async UniTask Initialize(IStateTransitionData data)
    {
        TransitionData = (TData)data;
        LifecycleCts = new CancellationTokenSource();
        
        await LoadView();
        await SetupView();
        await OnInitialized();
        
        View.SetActive(true);
    }
    
    public async UniTask Shutdown(bool applicationExiting)
    {
        if (_isShuttingDown) return;
        _isShuttingDown = true;
        
        // Cancel pending operations
        LifecycleCts?.Cancel();
        
        if (!applicationExiting)
        {
            await View.PlayExitAnimation();
        }
        
        await OnShutdown(applicationExiting);
        
        ReleaseView();
        
        LifecycleCts?.Dispose();
        LifecycleCts = null;
    }
    
    public virtual async UniTask Suspend(IStateTransitionData nextStateData)
    {
        View.SetInteractable(false);
        await OnSuspended();
    }
    
    public virtual async UniTask Resume()
    {
        View.SetInteractable(true);
        await OnResumed();
    }
    
    protected abstract UniTask OnInitialized();
    protected abstract UniTask OnShutdown(bool applicationExiting);
    protected virtual UniTask OnSuspended() => UniTask.CompletedTask;
    protected virtual UniTask OnResumed() => UniTask.CompletedTask;
    
    public abstract IStateTransitionData GetTransitionData();
}

Hierarchical State Machine

HSM Implementation

public class HierarchicalStateMachine
{
    private readonly Dictionary<string, IState> _states = new();
    private readonly Dictionary<string, string> _parentStates = new();
    private readonly Stack<string> _activeStateStack = new();
    
    public string CurrentState => _activeStateStack.Count > 0 ? _activeStateStack.Peek() : null;
    
    public void AddState(string id, IState state, string parentId = null)
    {
        _states[id] = state;
        if (parentId != null)
            _parentStates[id] = parentId;
    }
    
    public async UniTask TransitionTo(string targetState)
    {
        // Find common ancestor
        var currentPath = GetStatePath(CurrentState);
        var targetPath = GetStatePath(targetState);
        
        var commonAncestor = FindCommonAncestor(currentPath, targetPath);
        
        // Exit states up to common ancestor
        while (_activeStateStack.Count > 0 && _activeStateStack.Peek() != commonAncestor)
        {
            var stateId = _activeStateStack.Pop();
            if (_states.TryGetValue(stateId, out var state))
            {
                await state.Exit(false);
            }
        }
        
        // Enter states from common ancestor to target
        var enterStates = new List<string>();
        var current = targetState;
        
        while (current != null && current != commonAncestor)
        {
            enterStates.Insert(0, current);
            _parentStates.TryGetValue(current, out current);
        }
        
        foreach (var stateId in enterStates)
        {
            _activeStateStack.Push(stateId);
            if (_states.TryGetValue(stateId, out var state))
            {
                await state.Enter(null);
            }
        }
    }
    
    private List<string> GetStatePath(string stateId)
    {
        var path = new List<string>();
        var current = stateId;
        
        while (current != null)
        {
            path.Insert(0, current);
            _parentStates.TryGetValue(current, out current);
        }
        
        return path;
    }
    
    private string FindCommonAncestor(List<string> path1, List<string> path2)
    {
        string common = null;
        var minLength = Math.Min(path1.Count, path2.Count);
        
        for (int i = 0; i < minLength; i++)
        {
            if (path1[i] == path2[i])
                common = path1[i];
            else
                break;
        }
        
        return common;
    }
}

State Machine Patterns

Transition Guards

public class GuardedStateMachine
{
    private readonly Dictionary<(string from, string to), Func<bool>> _guards = new();
    
    public void AddGuard(string fromState, string toState, Func<bool> guard)
    {
        _guards[(fromState, toState)] = guard;
    }
    
    public bool CanTransition(string from, string to)
    {
        if (_guards.TryGetValue((from, to), out var guard))
        {
            return guard();
        }
        return true; // No guard = allowed
    }
    
    public async UniTask<bool> TryTransitionTo(string targetState)
    {
        if (!CanTransition(CurrentState, targetState))
        {
            Debug.Log($"Transition from {CurrentState} to {targetState} blocked by guard");
            return false;
        }
        
        await TransitionTo(targetState);
        return true;
    }
}

Transition Actions

public class StateMachineWithActions
{
    public event Action<string, string> OnStateTransition;
    public event Action<string> OnStateEntered;
    public event Action<string> OnStateExited;
    
    public async UniTask TransitionTo(string targetState)
    {
        var previousState = CurrentState;
        
        if (_states.TryGetValue(previousState, out var exitingState))
        {
            await exitingState.Exit(false);
            OnStateExited?.Invoke(previousState);
        }
        
        CurrentState = targetState;
        OnStateTransition?.Invoke(previousState, targetState);
        
        if (_states.TryGetValue(targetState, out var enteringState))
        {
            await enteringState.Enter(null);
            OnStateEntered?.Invoke(targetState);
        }
    }
}

State History

public class StateMachineWithHistory
{
    private readonly Stack<string> _history = new();
    private const int MaxHistorySize = 20;
    
    public bool CanGoBack => _history.Count > 0;
    
    public async UniTask TransitionTo(string targetState)
    {
        if (CurrentState != null)
        {
            _history.Push(CurrentState);
            
            // Limit history size
            while (_history.Count > MaxHistorySize)
            {
                // Remove oldest entries (need to convert to list)
            }
        }
        
        await DoTransition(targetState);
    }
    
    public async UniTask GoBack()
    {
        if (!CanGoBack) return;
        
        var previousState = _history.Pop();
        await DoTransition(previousState);
    }
    
    public void ClearHistory()
    {
        _history.Clear();
    }
}

Cleanup Patterns

Safe Async Disposal

public class SafeStateController
{
    private CancellationTokenSource _cts;
    private bool _disposed;
    
    public async UniTask Initialize()
    {
        _cts = new CancellationTokenSource();
        await StartBackgroundTasks(_cts.Token);
    }
    
    public async UniTask Shutdown(bool immediate)
    {
        if (_disposed) return;
        _disposed = true;
        
        // Cancel all pending operations
        _cts?.Cancel();
        
        if (!immediate)
        {
            // Give time for graceful cancellation
            try
            {
                await UniTask.Delay(100, cancellationToken: default);
            }
            catch { }
        }
        
        _cts?.Dispose();
        _cts = null;
    }
    
    private async UniTask StartBackgroundTasks(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            await DoWork(ct);
            await UniTask.Delay(1000, cancellationToken: ct);
        }
    }
}

Force Hidden State Pattern

public interface IForceHideable
{
    void ForceHiddenSuspend();
    void ReleaseForceHidden();
}

public class ActivityState : IState, IForceHideable
{
    private bool _isForcedHidden;
    
    public void ForceHiddenSuspend()
    {
        if (_isForcedHidden) return;
        _isForcedHidden = true;
        
        View.SetActive(false, immediate: true);
    }
    
    public void ReleaseForceHidden()
    {
        if (!_isForcedHidden) return;
        _isForcedHidden = false;
        
        View.SetActive(true, immediate: true);
    }
}

Best Practices

1. Guard against concurrent navigation - Use flags/locks
2. Wait before scope disposal - Allow pending callbacks
3. Use CancellationToken - For all async state operations
4. Track disposed state - Prevent double-disposal
5. Separate sync/async exits - applicationExiting parameter
6. Log state transitions - For debugging
7. Use typed transition data - Type-safe state parameters
8. Implement proper cleanup - Release resources in Exit
9. Handle navigation timeout - Don't deadlock on navigation
10. Test state transitions - Cover edge cases

Troubleshooting

Issue	Solution
Navigation deadlock	Add timeout to WaitUntil
Double disposal	Track disposed flag
Callbacks after exit	Use CancellationToken
Scope disposed early	Wait after Exit before Dispose
Lost state on crash	Implement state persistence